Integrated Optics - Applied Optics Fundamentals and Device Applications - page 250

exposure causes polymerization of the material making the exposed regions
insoluble. In positive acting resists exposure causes depolymerization, mak-
ing the exposed areas soluble and thus removable in suitable solvents.
Both resists have some advantages. Negative resists tend to be denser, stron-
ger coatings which stand up better to harsh processing steps. This is because
the exposure causes the formation of an interpenetrating polymer network
with a much greater degree of crosslinking than in the positive resists.
Positive resists generally produce slightly better resolution. This is because
they do not suffer the slight shrinkage resulting from crosslinking, nor are
they prone to swelling in the developing solutions as with negative resists.
For the standard Ti diffused LiNbO 3 waveguides positive resists tend to
dominate; however, for some of the newer processes which require a photo-
resist with greater chemical resistance the negative resists are preferred. Line
width control is still a major concern but advances in negative resist chem-
istry put them almost on a par with positive. Limitations for negative resists
under optimal conditions are currently 2.0 μm while for positive resists they
are 0.8 μm. Resolution is critical in Ti:LiNbO 3 devices in that control and defi-
nition of channel width are key to proper waveguide functionality. This is
especially true for devices such as Bragg cells where diffraction efficiency is
a direct function of waveguide geometry control.
Traditionally both types of resists have required exposure through a suit-
able patterned artwork or phototool. These phototools contain the desired
circuitry or waveguide pattern dependent on the resist type being used.
The phototools are typically made of chrome on glass substrates, are very
stable with respect to temperature and humidity, and are quite capable of
providing the degree of resolution required. They are, however, fragile and
expensive as well as time consuming to produce. Often the production of
hard tooling can be the most costly and time consuming part of the manu-
facturing process. An emerging technology that may eliminate the need for
phototools is that of direct write lithography. Due to their complexity and
density all of the required patterns for producing waveguides are currently
generated using computer automated design. This design is then translated
from the computer to produce the phototool which is then used to image the
pattern onto the substrate. All of these steps represent costs and potentials
for error and losses in resolution due to transferring the image from one
source to another. In direct write lithography the computer containing the
artwork pattern controls a suitable exposure source which is used to directly
write the desired pattern onto the photoresist coated substrates without the
use of a phototool. In theory these systems could reduce the costs and turn-
around times dramatically of prototype, custom, and short run devices by
eliminating the need to fabricate hard tools.
There are two methods which may be used to write images: raster scan
and vector scan. In raster scan the exposure beam is scanned back and forth
across the entire substrate. The pattern is written as a series of pixels by turn-
ing the beam on and off as required to define the pattern. In vector scanning